Photonic fiber-optic probes or sensors are useful in the measurement of RF fields because they minimally perturb the fields being measured, have low volume and weight, extensive bandwidths, and provide minimal thermal expansion of the signal transmission path, which facilitates reliable phase measurements. Prior fiber-optic probes have required both an expensive (usually high-power, low noise, and expensive-to-fiber-couple-to) laser and a separate sensing transducer. The transducer is typically an electro-optic modulator, for probes which probe electric fields, or a Faraday-effect modulator for probes which probe magnetic fields. These prior probes can be subject to calibration drifts due, for example, to temperature effects or fiber bending, because they include no means of checking or monitoring the sensitivity of the probe to RF fields while in operation. In order to avoid the use of field-perturbing metal wires or large and perturbing batteries, which must be replaced or recharged, these prior probes, further, have not been able to take advantage of modulators or directly-modulated lasers which require electrical biases (with one exception: U.S. Pat. No. 5,389,782 which employs an electro-optic modulator and an optically-powered amplifier). These prior-art probes further typically require expensive polarization-maintaining fibers which can also cause calibration drifts due to polarization cross-talk.
A prior voltage probe, as opposed to a field probe of the type shown in U.S. Pat. Nos. 5,583,444 and 5,703,491 used a directly-modulated laser in the probe head. This probe was limited to the capacitive pick up of voltages generated in devices on a surface (integrated circuits in particular) and to embodiments utilizing a constant current source to bias the lasers or which included placing and positioning means. No antenna capable of picking up free-space RF fields was involved--only voltage "detectors" capable of capacitively coupling to voltages relative to ground generated on a surface. The bias power was not provided optically, so metal conductors (which would grossly perturb free-space RF fields) were required. Further, because a constant current source was used, variations in temperature could cause significant changes in probe sensitivity by changing the slope of the laser power vs. laser input current curve.
A fiber-optic link (U.S. Pat. No. 5,739,938), which is not an RF field probe, but which could potentially be used as such if connected to an appropriate antenna, used a directly-modulated laser and an optically-supplied DC power source. The transmitter (main part of the probe head in a probe configuration) of this link was limited, however, by the inclusion of a PIN photodiode in the "laser module" together with a laser power regulator in the transmitter portion in all embodiments. These parts require a relatively large amount of power. The inclusion of this circuitry in the transmitter increases its size, weight, and electrical power consumption. Moreover, since this power was supplied by inefficient optical means (normally involving the loss of around 150% of the power consumed in the optical-to-electrical power converter alone), the total power consumption is very high. Moreover, much of this power must be dissipated within the transmitter itself, leading to thermal management complexities. The increased transmitter size, weight, and power consumption are problems in probes used for measurement purpose and, especially, in applications such as phased-array radars, where a very large number of such transmitters are involved. In addition, while the PIN photodiode and power regulator are used to keep the light power generated by the laser constant, there is no provision made to compensate for changes in the slope of the laser power vs. laser input current curve or in fiber coupling or transmission losses-the parameters on which the probe or link sensitivity is dependent. These parameters can change with environmental effects such as temperature independently of laser power. Furthermore, the many different parts, including the PIN photodiode, the power regulation circuit, and parts used for fiber coupling, are exposed to these same environmental effects possibly including, in addition to temperature, vibration, dust, humidity, and electromagnetic noise. This link is also limited to transmitters which incorporate an RF amplifier, which adds further complexity, volume, weight, and inefficient power consumption, as well as further susceptibility to various environmental effects, and is not needed in many probe applications.